Earth is Round

How We Know the Earth is Round? and Other Related Questions

Objectives:

explain how astronomers answer some of simplest, yet most important questions
about the Earth's shape

explain the practical significance of a "round" Earth

advanced material explains why planets are "round"

Introduction

We've all been told that the Earth is round, but this is not something
our day-to-day experience can easily confirm. Looking outside, we see buildings,
trees, maybe some hills or valleys... very little evidence that the Earth's
surface is curved. Even when we're at the shore of a large lake or the
ocean, the water near the horizon appears quite flat. How then do scientists
know that the Earth is round?

The simplest and most convincing proof that the Earth's is "round" is
to examine a photo of our planet taken from space or to orbit it in the
space shuttle.

There can be no doubt from such evidence that the Earth is round, or
more correctly, a "sphere." The word "round" refers to a circular shape,
like the shape of a quarter when held flat in your hand. The proper term
to describe the Earth's shape is spherical. A sphere appears round when
viewed from any direction in space. It's okay to use the term "round" to
refer to the shape of the Earth, so long as you're aware of this distinction.

Basic Arguments

Scientists knew the Earth was "round" centuries before humankind ventured
into space. What evidence did they have for this? There are three good
pieces of evidence. The first dates back to at least 200 B.C. with the
Greek astronomer, Eratosthenes. He was aware that at noon on the summer
solstice or the first day of summer, the Sun in the city of Syene passed
directly overhead. Yet, on the same day in the city of Alexandria -- some
5000 stadia to the north -- vertical objects cast a shadow of 7 degrees.
If the Earth were flat, this observation is hard to understand. Yet if
the Earth were round (spherical), Eratosthenes reasoned, one could use
this information to compute the Earth's circumference. The situation is
illustrated in figure 2.

Now if 7 degrees subtends 5,000 stadia, then 360 degrees -- the circumference
of the Earth -- corresponds to just over 250,000 stadia. If 1 stadium was
0.16 km as we believe, then Eratosthenes obtained a value of 40,000 km
for the circumference of the Earth which is remarkably close to today's
accepted value. This underscores how powerful simple geometry can be to
learning about our world.

Have you ever seen a lunar eclipse? A lunar eclipse occurs when the
Earth comes between the Sun and the Moon causing the Moon to enter the
Earth's shadow. [show figure of lunar eclipse here] While not as spectacular
as a Solar eclipse (when the Moon comes between the Sun and the Earth),
the Moon turns a wonderful copper or blood-red colour. The important point
for us, however, is the shape of the Earth's shadow as it sweeps over the
Moon's surface; it is circular, exactly as one would expect if the Earth
were spherical.

A skeptic -- someone who needs more proof before accepting such an important
idea -- might protest that a sphere is not the only shape which casts a
circular or round shadow. And she would be right. For instance, a quarter
is certainly not spherical, but if held in the right way, casts a circular
shadow. How then can we convince her? The Earth's shadow on the Moon as
it enters or leaves a lunar eclipse is circular regardless from where it
is viewed; whether from Canada or Australia. Only a sphere casts a circular
shadow when viewed from any direction. A quarter when tilted or turned
on its side has a distinctly non-spherical shadow. Try it some time.

The other well known proof for the roundness of the Earth is often associated
with Christopher Columbus who first sailed from Europe to North America
in 1492. At that time, many people in Europe, including the well educated,
believed that the Earth was flat. Common sense told them this. If one sailed
too far in one direction, one would fall over the "edge." Columbus thought
that the Earth was round (spherical) and gave the following proof: when
a large ship (with a tall mast) sails away from a port, it appears to get
smaller and smaller before it completely disappears from sight at the horizon.
The curious thing, however, is that while the entire ship is visible as
it leaves port, only the mast remains visible just before it vanishes.
This is a natural consequence of sailing on a spherical Earth as figure
3 shows.

When a large ship is far enough away, the body first disappears under
the horizon followed by the mast. Columbus was right!

There are other interesting proofs of the Earth's roundness which don't
require a trip into space, though they do require some careful planning!
Suppose you had several friends who lived in various cities across the
country; say Halifax, Montreal, Winnipeg, Regina, Calgary and Victoria.
You phone each of them up and ask them to phone you tomorrow the instant
they see the Sun rise on the eastern horizon. (Let's assume you have enough
money to pay for the phone calls, that your family doesn't mind getting
calls in the middle of the night, and that it is clear right across Canada
the morning of the experiment.) What would happen? Your first phone call
would come from your friend in Halifax, followed later by your friends
in Montreal, Winnipeg, Regina, Calgary and finally Victoria in this order
(and a few hours after your first). Each friend can confirm that he or
she is observing the same Sun since our Sun's surface is normally covered
in spots with a definite pattern which would be the same for all observers.
The simplest explanation for why the times of sunrise differ across the
country in this manner is because the Earth is round. (Of course, we all
know the Sun doesn't actually rise; rather it is the Earth turning on its
axis which gives the illusion of the Sun rising and setting.)

Experiment:

[You can demonstrate this effect by taking a globe of the Earth which rotates
on its axis. One student can play the role of the Sun by sitting in one
place or even holding a flashlight pointed at the globe. Another student
can slowly turn the Earth on its axis situated a couple of metres away
from the "Sun." In which direction, E-W or W-E, do you have to turn the
Earth on its axis in order for sunrise to occur in Halifax before Victoria?
One can add even more weight to the hypothesis that the Earth is round
by enlarging the circle of friends who participate in the experiment; eg.,
in Europe, Asia, etc. This is precisely why Sir Sanford Fleming, a Canadian
from the 19th century, suggested the concept of Time Zones so that the
Sun would be highest in the sky when the local time was noon.]

Why does "common sense" tell us the world is flat?

There is overwhelming evidence that the Earth is round. Why then do we
perceive it as flat? The answer is that the Earth's diameter is several
million times larger than the height of an average human. While standing
on the ground or even in a tall building, we're unable to see the curvature
of the Earth's surface because it is so gradual from our perspective. That's
why we must go to very high altitudes, and even space, before we can tell
that our planet is spherical. Think of placing an ant on an orange. The
ant, if it were intelligent, would certainly notice that its world was
curved. Now place the ant on a giant beach ball. It could probably tell
it occupied a curved world, but it would be somewhat harder for it to tell.
Finally, place the same ant on the roof of the SkyDome (when its closed).
The ant would now not be able to tell its world was curved at all; it would
seem like its world was flat, yet we know differently. The same principle
applies to us. Like the ant, the ball on which we live is much too large
for us to detect its curvature.

Travelling on a Spherical Earth

The truth is, we don't normally concern ourselves with the correct shape
of the Earth in our day-to-day life. In fact, the world may just as well
be flat when we go to school, visit a friend, or drive to a restaurant.
Even our maps make it appear like the Earth is flat. When it comes to air
travel, however, the shape of the world does matter. Flying from one continent
to another, it is essential that pilots take into consideration that our
world is spherical, not flat. Why is this? Airplanes fly at a constant
height above the ground, so if the Earth's surface is curved, the path
of the airplane will be curved. In order for an airplane to reach its destination
successfully and to use the least amount of fuel, the pilot must take into
consideration that the world is round.

You can easily demonstrate this by performing the following experiment.
You'll need three things; some string, a globe, and a flat world map. Suppose
you'd like to fly from Vancouver, British Columbia to London, England.
On the flat world map, locate both cities. Now take the string, fix one
end on Vancouver, and draw the string tight so that it also passes over
London. Record any major cities or bodies of water over which the string
passes on the map. Repeat the same process with the globe which represents
the spherical Earth. Place one end of the string at Vancouver, and find
the route over the Earth which uses the smallest length of string and which
terminates at London. Record any major cities or bodies of water over which
the string passes.

There is a radical difference between these two paths, isn't there?
Apart from the beginning and ending of the path, not much else is the same.
The path you've determined on the globe is very close to the actual flight
path a modern jet follows; it does get very close to the North Pole! The
world is round (or at least spherical) after all.

There are many interesting consequences of living on a spherical Earth.
One with religious significance can be illustrated with an experiment similar
to the one you've just performed. In the religion of Islam, Muslims are
encouraged to pray while facing towards Mecca, Islam's most holy city,
at certain times during the day. The question for Muslims who live in Canada
is, which direction do they turn to face Mecca? Repeat the experiment above,
this time using the cities of Toronto and Mecca (in Saudi Arabia). Since
Mecca is east of Toronto and much closer to the equator, some people believe
that one should face south-east. But the globe experiment will quickly
convince you that the proper direction to face is actually north-east!

Why is the Earth Round (advanced)?

Not only is the Earth spherical in shape, but so too is our nearest neighbour,
the Moon. Have you ever looked at the Moon through binoculars? If you have,
you'll probably recall a number of interesting things. For instance, you
may have noticed that the Moon is mostly composed of light-coloured material
with lots of craters, and some dark patches with fewer craters. But think
even harder. You probably also noticed that its "edge" seems to curve around
and then out of sight when it reaches the darkness of space, just like
a giant ball. In other words, the Moon, too, is spherical.

Photos of Mars and other nearby planets from the Hubble Space Telescope
also show these bodies to be spherical, just like pictures of the Earth
taken from the Moon or space. (In fact, one can watch them rotate on their
axis.) Do you recall seeing a picture of an asteroid, a chunk of rock leftover
from the formation of the Solar system and which can be the size of a large
city (the largest is the size of the distance between Halifax and Ottawa).
Asteroids are far from spherical in shape, looking like pieces of driftwood.
Have you ever wondered why all the planets and our Sun are spherical, while
smaller bodies are not?

When it comes to energy, nature doesn't like to be wasteful. It much
prefers to have bodies in the lowest possible energy state. This is true
for atoms as well as planets. Planets are spherical for two reasons; first,
a sphere is the shape which has the lowest energy for a given size so far
as gravity is concerned, and second, planets are massive enough that their
gravity can force material to change its shape over a long period of time.
In the case of a rock or an asteroid, gravity is not strong enough to force
it to become spherical from its original shape.